While the invention relates to the testing of wooden structural elements in general, one common example of a structural element that needs monitoring is the utility pole. Utility poles are typically a length of the trunk of a pine tree. After cutting, the poles are seasoned for sufficient time to dry and stabilise the timber, and are then treated with preservative chemicals. The poles are intended to have a pre-determined length buried in the ground, to provide sufficient structural stability. In some conditions, such poles can remain serviceable for many years—some have remained sound for several decades.
In some conditions, and/or where treatment of the timber has been insufficient after cutting, rot can occur, substantially reducing the safe working life of the pole. When work is to be done on cables supported by the pole, for example, it is typically acceptable for a ladder to be leant against the pole to support the person doing the work. Before work commences, it is necessary to test the state of the pole to ensure that it has sufficient strength. The simplest form of testing has been carried out by striking the pole with a hammer and listening to the resultant sound. With careful training, a person can distinguish by listening to this resultant sound between a sound pole and one which has been weakened by decay. If the pole is deemed unsafe for ladder access, the work may need to be carried out from an access hoist, which involves extra cost and possible delay, but it may become necessary for the pole to be replaced, which is even more costly. It is therefore important to ensure that testing is as accurate as possible, and accuracy is difficult to achieve where the test criteria are subjective.
There have been proposals for mechanically inducing acoustic energy into the pole, recording the output from an audio transducer attached to the pole, and analysing the output to provide an indication of the state of the timber. Earlier techniques simulated the manual testing technique, with a high energy blow to the pole serving to cause the pole to “ring” at the resonant frequencies thereof, with the transducer then picking up the response. This is imprecise, because a wide band of acoustic frequencies is generated simultaneously with little or no control of the spectral profile of the input acoustic energy, so initiating resonances due the excitement of other waveforms. Furthermore, the high-energy pulses required, typically delivered manually, to generate sufficient acoustic power cause local damage to the structure. In some instances, the results obtained were no more accurate than could be obtained by a trained person listening to the sound.
U.S. Pat. No. 4,399,701 discloses a system involving the generation of a continuous longitudinal acoustic wave by inserting transducers (for example piezoelectric transducers) into slots cut into the pole at locations spaced apart along is its length. The input acoustic frequency is varied over a range that includes, say the first twenty standing wave harmonic frequencies of the pole. If the output contains harmonic resonances, then this can be taken as an indication of a sound pole, while a pole containing rot is said to yield resonant frequencies which are not in harmonic relationship with one another. It is not explained how the judgment on these issues is made.
A major disadvantage of this technique is that it necessitates the cutting of slots into the pole, something that utilities operators would consider highly undesirable, since it would tend to weaken the pole and to provide a possible future route into the interior of the pole of damaging moisture and attacking organisms. It is therefore desirable to provide a testing method which is non-invasive and which provides a more accurate indication of the state of the wood in the pole.